1. Field of the Invention
The present invention generally relates to a method for measuring thickness of material by ultrasound and, in particular, relates to a method for improving the correlation between "Ultrasonic measurements" and "mechanical measurements" by a priori knowledge of surface roughness.
2. Description of the Prior Art
An ultrasonic thickness gauge (UTG) is used to measure the thickness of materials such as metal, ceramics and glass by the time measurement of the temporal delay between two reflected acoustic pulses which emitted from a common source, where the first pulse is the reflection from the first (or near side) surface and the second pulse is the reflection from the second (or far side) surface.
The electronics which executes a single instrument reading can be visualized to have four basic functions: the pulse generator that initiates the sonic pulse; the delay circuits that time aperture a detection circuit; the echo pulse detection circuits that initiate and terminate a pulse counter; and an independent clock that generates the pulses that are summed by the counter. Nominally a 10 nanosecond (ns) clock is used.
Since the "pulse generator and detection circuit and counter and clock" are asynchronous, the number of 10 ns pulses summed within a fixed detection window will vary by .+-.0.5 pulses even in an ideal noise-free environment.
Since the time variance of .+-.0.5 pulses is random, the variance of the reported reading is correctly reduced by an average of many samples which constitutes a single reading. Thus variance of the averaged value is now reduced by the square root of the number of "n", where "n" is the number of samples in a single reading. It should be noted, that if "n" is properly selected such that the variance of the readings is larger than the quantization of the instrument, subsequent averaging of the readings may reduce the variance of the reported measurement (time or thickness) by the square root of "m", where "m" is the number of readings that are averaged in the reported measurement.
Empirical data acquired by the inventors have demonstrated that the variance of many successive measurements does follow the common statistical laws where the standard deviation of a measurement (.sigma..sub.m) is equal to the standard deviation of a sample (.sigma..sub.s), reduced by the square root of the number of samples in that measurement (m.times.n), that is: ##EQU1## However, in the averaging of this large number of samples, the true window will be reported with a negative 0.5 pulse bias (that is -5 ns for a 10 ns clock).
The time between the first and second pulse is assumed to be the total distance traveled divided by the velocity of the acoustic pulse in the media that is being measured. The thickness is then assumed to be 1/2 the increased distance that the second reflected sound pulse traveled relative to the first reflected sound pulse since the second reflected pulse passed though the media twice in addition to the identical paths of the first reflected pulse. Thus:
(2) time between pulses=differential distance/velocity=(thickness)(2)/velocity, and PA1 (3) Thickness=1/2 (time)(velocity)=(time)(1/2 velocity)=(time)(Va), PA1 where: PA1 time=the measured elapsed time (here after referred to as time of flight), and PA1 Va=one half of the temperature corrected sonic velocity of the sample (material or media). PA1 t'=the measured time of flight of the calibration sample, PA1 h'=the mechanically measured thickness of the calibration sample (material or media), PA1 Va'=one half of the temperature corrected sonic velocity of the calibration sample (material or media), and PA1 C(f)=the surface finish dependent time offset (Correction is required to establish the correlation for all values of (h') and (t') as a function of surface conditions. These values include any time measurement bias from the measurement clock and the function of roughness for each surface or the method or process or operation that prepared each surface). PA1 t=the measured time of flight, PA1 Va=one half of the temperature corrected sonic velocity of the sample (material or media), and PA1 C(f)=the surface finish dependent time offset previously discussed in equation (4).
It should be noted that the above equations do not account for any affect that are correlated to, or a function of, surface preparation or surface roughness and it should be noted that the equation does not account for any time bias.
Likewise, experience states that the surface finish of measured material are usually not the same, i.e. a glass sample that is rough ground and later when it is polished, or a plate steel that is rough milled versus ground or polished.
Accordingly, one disadvantage of the previous technology is that the UTG system does not measure the true elapsed time between two pulses (hence thickness of a sample when the true sonic velocity is used in the computations). Likewise, another disadvantage is that the change in a sample's thickness is incorrect when the surface roughness was altered between the two comparative measurements. That is, if the UTG were used as an in-process monitoring device to determine material removal as the surface finish were altered, the subsequent absolute mechanical measurements would not agree.